You can make sure the metadata is correct, before you import the book to Mantano. Or do as I described above, to minimize the inconvenience. However, if you update the metadata from Mantano, then it is changed ok. But only the copy on the device. Not the metadata in the Calibre library, obviously.
Yes, this is annoying. It would be very nice if you could update books without having to remove them and reimport. Obviously if the book change name, then Mantano can no longer keep track of the book.
But I still think Mantano is the best ereader app I have tested.
I also know that representatives from Mantano seems to keep an eye on this site. Perhaps they can add a metadata rescan?